Add .rspec and require 'helper' from there

Just discovered this nice feature of RSpec where it can load all files for
us. Works with `bundle exec rake` and `bundle exec rspec spec/file_spec.rb`,
which covers all use cases.

spec/config/behavior_spec.rb

@@ -1,4 +1,3 @@
-require 'helper'
 RSpec.describe Pry::Config::Behavior do
   let(:behavior) do
     Class.new do

spec/config/memoization_spec.rb

@@ -1,4 +1,3 @@
-require 'helper'
 RSpec.describe Pry::Config::Memoization do
   let(:config) do
     Class.new do